Nils Jansson

Timing of volcanism, hydrothermal alteration and ore formation at Garpenberg, Bergslagen, Sweden

The timing of Palaeoproterozoic magmatism in the Garpenberg area in the Bergslagen region of the Fennoscandian shield has been constrained by secondary ion mass spectrometry (SIMS) U–Pb zircon dating of metamorphosed igneous rocks. Volcanism is constrained by igneous crystallisation ages of 1895 ± 4 Ma for a syn-volcanic rhyolite porphyry intrusion and 1893 ± 3 Ma for a rhyolitic pumice breccia. Granite and microgranodiorite, which intruded into the stratigraphy, are dated at 1895 ± 3 and 1894 ± 4 Ma, respectively. The identical U–Pb ages suggest rapid geological evolution from the emplacement of volcanics, their burial and subsidence to 2–5 km depths and intrusion by granitoids. The timing of metamorphism and the extent of metamorphic resetting of titanite have been evaluated. SIMS titanite 207Pb–206Pb ages from the same samples as the zircon yield younger ages. Although errors are large in individual analyses and fractions, a weighted average of 59 analyses from four samples yields a 207Pb–206Pb age of 1858 ± 14 Ma, interpreted as the age of regional metamorphism. The results add constraints to the timing of sulphide and iron oxide mineralisation at Garpenberg. The rhyolite porphyry is intruded into a syngenetic iron formation. Its crystallisation age provides a minimum age for syngenetic iron oxide deposits at Garpenberg. The major Zn–Pb sulphide deposits are accompanied by alteration envelopes. Units formed before alteration yield similar igneous crystallisation ages as intrusions post-dating alteration. It is concluded that both iron oxide and sulphide mineralisation formed within the same age-span as the dated units.

Structural evolution of the Palaeoproterozoic Sala stratabound Zn-Pb-Ag carbonate-replacement deposit, Bergslagen, Sweden

Abstract A structural investigation of the Sala Zn–Pb–Ag deposit in the Bergslagen mining district of southern Sweden shows that it is associated with two tectonic structures: the N–NW-trending Storgruveskölen shear zone (SSZ), which is parallel to the strike of the mined ore bodies, and the F1 Sala syncline with a fold hinge plunging c. 35° towards NNW, which is parallel to the plunge of the entire mineralised system. The Sala syncline was refolded by F2 folds, leading to flattening and local reversals in the plunge of F1 folds and the ore bodies. Field evidence suggests that the SSZ represents both a phase of D3 reverse dip-slip shearing and a later (D4) phase of dextral strike-slip reactivation. However, a high concentration of pre- to syn-D1 skarn- and sulphide-bearing vein networks and breccias adjacent to the SSZ, which are gradational into the mined massive sulphide ore bodies, suggest that stages in the formation of the SSZ predated D3 and D4. It may consequently constitute a reactivated pre- to syn-D1 structure. The distribution of breccia and hydrothermal alteration together with the highly discordant nature of the deposit are consistent with a pre- to syn-D1 timing of ore formation, involving of cross-stratal fluid flow along the proto-SSZ and subordinate fluid flow parallel to volcanic interbeds in the host carbonate rock. Three δ34S determinations on sphalerite (2.1–2.4‰) and galena (1.2‰), respectively, are consistent with a magmatic source for ore sulphur, as has been suggested for many other sulphide deposits in Bergslagen.

A comment on the occurrence of gallium and germanium in the Zinkgruvan Zn-Pb-Ag-(Cu) sulphide deposit, Bergslagen, Sweden

Abstract The Zinkgruvan deposit has been included in compilations of exceptionally Ga- and Ge-endowed deposits in Sweden. Available published data sets do, however, not support a substantial enrichment in Ga and Ge. In this contribution, we investigate the Ga- and Ge-endowment based on a whole-rock lithogeochemical data and ore grade analyses from the deposit. Based on our results, we find it highly unlikely that a Ga-endowment exists in the ore. A Ge-endowment may exist, but we find no evidence of Ge grades at the 1000 ppm level that have been reported previously.